1. Field of the Invention
The invention generally relates to a lens, and more particularly, to a fixed-focus lens module.
2. Description of Related Art
In recent years, digital display apparatuses such as liquid crystal displays (LCDs), digital light processings (DLPs), and plasma display panels (PDPs) have gradually replaced conventional cathode ray tubes (CRTs), and are widely adopted in new display apparatuses. Since digital video signals can be transmitted and processed without distortion, applying digital display apparatuses in projection display apparatuses (such as rear projection digital display apparatuses) has become the mainstream in the market. However, adopting lens modules in projection display apparatuses is challenging for optical designers.
For example, in order to represent superior imaging quality, the lens module used in conventional projection display apparatus usually has features of small distortion and image aberration, high definition, high contrast ratio, and uniform luminance. Moreover, these high quality projection display apparatuses not only have superior imaging quality, but also include larger viewing angles to satisfy the requirement for projecting large frames in a small space. Furthermore, in order to enhance light source utilization and frame luminance uniformity, the main light beam at the reduced side of the lens module and the optical axis thereof should be as parallel to each other as possible. That is, a telecentric angle of the main light beam at the reduced side relative to the optical axis should be as small as possible.
An optical module disclosed in Taiwan Patent No. M362997 includes a front lens group and a rear lens group. The front lens group includes a first lens, a second lens, and a third lens. The first lens has a biconcave shape. The second lens has a positive refractive power and a lens surface on an image side is a convex surface. The third lens has a positive refractive power and a lens surface on an object side is a convex surface. The rear lens group includes a fourth lens and a fifth lens sequentially from the object side. The fourth lens has a negative refractive power and a lens surface on the image side is a concave surface. The fifth lens has a biconcave shape. The optical module has high environment tolerability.
In FIG. 1 of U.S. Pat. No. 6,747,816, a four-piece wide-viewing angle lens is disclosed. The refractive powers of the four-piece lens from the object side to the image side are negative, positive, negative and positive sequentially. Herein, a surface of a first piece of lens facing the object side is a concave surface, and at least one surface of a second piece of lens is an aspherical surface. A surface of a fourth piece of lens facing the image side is a convex surface, and at least one surface of the fourth piece of lens is an aspherical surface. In addition, a two-group four-piece zoom lens is disclosed in FIGS. 8 and 9 of U.S. Pat. No. 4,993,814. This zoom lens includes a first lens group (a lens having a negative refractive power) and a second lens group (assembled by lenses having positive, negative and positive refractive powers respectively). An aperture stop is located between the first lens group and the second lens group.
A two-group five-piece zoom lens disclosed in FIG. 1 of U.S. Pat. No. 5,233,474 includes a first lens group and a second lens group. An aperture stop is located between a third piece of lens and a four piece of lens of the second lens group, and a surface of the four piece of lens facing the magnified side is an aspherical surface. Furthermore, a two-group five-piece wide-viewing angle lens disclosed in FIG. 6 of U.S. Pat. No. 5,796,528 includes a first lens group and a second lens group. An aperture stop is located between the first lens group and the second lens group. Similarly, a two-group lens is disclosed in U.S. Pat. No. 5,412,508 and U.S. Pat. No. 4,679,913. A four-piece fixed-focus lens is disclosed in US Patent Publication No. 20030161050.